Cellular senescence, a permanent cell cycle exit triggered by different stresses, has recently emerged as a safeguard mechanism against both uncontrolled proliferation and the accumulation of deleterious genetic alterations that occur during oncogenic transformation. Markers of cellular senescence have been identified in early stage human cancers, including preneoplastic prostate lesions, but are lost as the tumors progress. Consistent with a role of senescence in the prevention of tumor progression, genetic inactivation of essential components of the senescence pathway in mouse models leads to the acceleration of cancer progression. Despite accumulating evidence for its biological relevance in tumor suppression, the molecular bases underlying the establishment of cellular senescence remain largely elusive. Recently, the transcriptional silencing of pro-proliferative genes via heterochromatinization has been shown to correlate with the permanent senescence-associated cell cycle exit. Our preliminary results using genetically engineered mice and cells, demonstrate that the histone deacetylase (HDAC) associated Sin3B protein is required for both replicative and oncogene-induced cellular senescence. In addition, Sin3B is specifically induced upon oncogenic stress, and its overexpression is sufficient to induce cellular senescence in primary fibroblasts. The specific aims of this proposal include the determination of the underlying molecular and cellular mechanisms by which Sin3B regulates senescence and prevent cancer progression in mammals. Specifically, we propose to identify the molecular events leading to Sin3B upregulation upon oncogenic stress, and determine how Sin3B upregulation induces cellular senescence (Aim 1); to investigate the contribution of Sin3B-induced senescence in the suppression of cellular transformation in fibroblasts (Aim 2); to test the hypothesis that Sin3B expression prevents prostate tumor progression in vivo (Aim 3). To do so, we will use a combination of molecular, cellular and biochemical approaches, as well as genetically engineered mouse models of cancer.